Gas turbine engines operate at temperatures that are disruptive to the natural characteristics of metal and other engineering materials such as ceramics and composites. Such conditions cause material to fatigue, stress and fail. It is desired to provide stress-relief features by providing slots and various other geometric configurations in the surface of a material, such as a material for use in a turbine liner, and as they appear in auxetic structures.
While virtually all materials undergo a transverse contraction when stretched in one direction and a transverse expansion when compressed, auxetic materials do not. The magnitude of the transverse deformation exhibited by materials upon compression or stretching is expressed by a quantity known as Poisson's ratio. In ordinary materials that contract laterally when stretched and expand laterally when compressed, Poisson's ratio is defined as a positive number. However, some materials, when stretched, become thicker in the direction perpendicular to which they are being stretched. Such materials have a negative Poisson's ratio, and are referred to as auxetic materials.
The structure of a material may be altered in such a way that the material exhibits auxetic behavior. One way in which this may be done is by disposing an exemplary pattern of elliptical holes within and extending through the plane of the material. However, materials that are modified to exhibit auxetic behavior in this manner may exhibit stress concentrations at the edges of the minor radii of the holes. The stress concentrations may lead to cracking and, in severe cases, component failure. A need exists for a material that exhibits auxetic properties, and that will not be subject to stress concentrations and cracking.